OXIDATIVELY INDUCED ALKYNE ROTATION IN DICOBALT COMPLEXES - STRUCTURAL TESTS OF MOLECULAR-ORBITAL THEORY

The complexes [Co2(CO)2(mu-RC2R)(mu-dppm)2] 1 (R = Me, Ph or CO2Me; dppm = Ph2PCH2PPh2) undergo reversible one-electron oxidation at a platinum-bead electrode in CH2Cl2; chemical oxidation of 1 with the ferrocenium ion or with iodine gives stable salts of the monocations 1+. The X-ray structures of 1 (R = Me and Ph) show the alkyne bridge to be orthogonal to a cobalt-cobalt bond which is also bridged by the two dppm ligands. The overall structure of 1+ (R = Me), as its [PF6]-salt, is similar but with the alkyne bridge rotated by 12-degrees about an axis perpendicular to the shortened metal-metal bond. The structural changes observed provide direct experimental support for previous bonding studies of the structural preferences of hydrocarbon-bridged M2(CO)6 complexes.